The present invention relates to a magnesium oxide particle aggregate having a controlled particle aggregation structure. More particularly, the present invention relates to a magnesium oxide particle aggregate used as an annealing separator to form a forsterite film which imparts excellent insulation properties and magnetic properties to a grain-oriented magnetic steel sheet.
Grain-oriented magnetic steel sheets used in transformers or generators are generally produced by a process in which silicon steel containing about 3% of Si is hot-rolled, subsequently cold-rolled so as to have a final sheet thickness, and then subjected to decarburization annealing (primary recrystallization annealing), followed by finishing annealing. In this process, for imparting insulation properties to a magnetic steel sheet, after the decarburization annealing and before the final finishing annealing, a slurry containing magnesium oxide is applied to a surface of the steel sheet and then dried, and wound into a coil form. Si contained in the silicon steel reacts with oxygen during the decarburization annealing to form a SiO2 film on the surface of the steel sheet. SiO2 in the film then reacts with magnesium oxide in the slurry during the finishing annealing to form a forsterite (Mg2SiO4) film having excellent insulation properties on the surface of the steel sheet. The forsterite film is considered to impart not only insulation properties but also a tension to the surface thereof due to the difference in the coefficient of thermal expansion between the forsterite film and the steel sheet, thus lowering core loss of the grain-oriented magnetic steel sheet to improve the magnetic properties.
Therefore, the forsterite film plays an extremely important role in the production of grain-oriented magnetic steel sheets, and hence the properties of magnesium oxide forming the forsterite film directly affect the magnetic properties thereof. For this reason, there have conventionally been demands for magnesium oxide used as an annealing separator to meet the requirements of excellent properties and resultant precise control. In view of this, a number of inventions have been made with respect to the magnesium oxide used as an annealing separator.
One example of such inventions is to add an additive to magnesium oxide or to control an impurity content thereof. For example, with respect to magnesium oxide utilizing an additive, Japanese Patent Publication No. 45322/1995 (process for producing a magnesium oxide composition) discloses a process for producing magnesium oxide, in which a predetermined amount of a boron compound is added to Mg (OH)2 containing chlorine and then calcined under a predetermined steam partial pressure.
On the other hand, many inventions in respect of the activity determined by the reaction rate between magnesium oxide particles and an acid, i.e., citric acid activity (CAA) are also disclosed. CAA is represented by a period of time required until a 0.4 N aqueous solution of citric acid at a predetermined temperature (for example, 22xc2x0 C. or 30xc2x0 C.) containing phenolphthalein as an indicator becomes neutral from a point in time when a final reactive equivalent amount of magnesium oxide is added to the solution and stirred. It is empirically known that CAA can be used as an index for evaluation of the magnesium oxide used as an annealing separator for the grain-oriented magnetic steel sheet.
As an invention in respect of the distribution of CAA at a reactive equivalent amount of magnesium oxide, Japanese Prov. Patent Publication No. 58331/1980 discloses an invention of magnesium oxide for an annealing separator having an activity adjusted so that the distribution of CAA is controlled in a narrow range for each final reaction rate of 20%, 40%, 60% and 70%, respectively. In addition, Japanese Prov. Patent Publication Nos. 33138/1994 and 158558/1999 disclose an invention of magnesium oxide for an annealing separator, in which the activity of CAA of 40% or 80%, the particle diameter and the specific surface area are restricted to respectively predetermined values. Further, Japanese Prov. Patent Publication No. 269555/1999 discloses an invention of an annealing separator for grain-oriented magnetic steel sheets, in which CAA of 70%, the ratio of CAA of 70% to CAA of 40%, the particle diameter, the specific surface area and the like are restricted to predetermined values, respectively. In each of the above inventions, the hydration property and the reactivity of the magnesium oxide particles are controlled.
The activity of magnesium oxide indicated by CAA is a yardstick for the reactivity in the solid phase-liquid phase reaction between magnesium oxide and citric acid. In this solid phase-liquid phase reaction, the larger the number of reactive sites in the solid phase, that is, the smaller the particle diameter of magnesium oxide or the larger the specific surface area thereof, the larger the surface free energy to increase the activity.
However, in powder particles including magnesium oxide particles produced by various methods, oxide particles can be existed in the form of particle aggregate in which several powder particles are bound together and agglomerated in addition to the case existed in the form of individual particle. In the particle aggregate caused by agglomeration or aggregation, the CAA measured is not a value reflecting the structure of the particle aggregate and therefore cannot precisely represent the reactivity of an annealing separator.
Further, CAA merely simulates empirically the reactivity in the solid phase-solid phase reaction, which actually proceeds between SiO2 and magnesium oxide on the surface of the magnetic steel sheet, using the solid phase-liquid phase reaction between magnesium oxide and citric acid. Differing from the solid phase-liquid phase reaction, in the forsterite formation reaction which is a solid phase-solid phase reaction, the particle aggregation structure of magnesium oxide, for example, the number of contact points between the SiO2 film and the magnesium oxide particles, is presumed to remarkably affect the reactivity. Specifically, even when the magnesium oxide particles have active surfaces, a small number of contact points derived from the particle aggregation structure cause the reaction to proceed unsatisfactorily. On the other hand, even when magnesium oxide particles have inactive surfaces, an increased number of contact points can advance the reaction satisfactorily.
As mentioned above, CAA used as an index of the properties of an annealing separator for the magnetic steel sheet is a yardstick for evaluation of the reactivity of magnesium oxide only under given conditions. It is considered that CAA dose not necessarily precisely evaluate the solid phase-solid phase reaction which actually proceeds on the surface of the magnetic steel sheet. Therefore, in magnesium oxide having a poor activity evaluated by CAA, there is a possibility that magnesium oxide having a particle aggregation structure suitable for an annealing separator can be found by using a method of controlling the solid phase-solid phase reaction taking into consideration the aggregation structure of powder particles.
In view of the above, an object of the present invention is to provide a magnesium oxide particle aggregate having a controlled particle aggregation structure so that the solid phase-solid phase reaction between magnesium oxide and the SiO2 film on the surface can be appropriately controlled. An object of the present invention is further to provide an annealing separator for a grain-oriented magnetic steel sheet, using the magnesium oxide particle aggregate of the present invention, and to provide a grain-oriented magnetic steel sheet obtainable by a treatment using the annealing separator of the present invention.
The present inventors have conducted extensive and intensive studies to solve the above-mentioned problems and thus completed the present invention. Specifically, the present invention is a magnesium oxide particle aggregate characterized in that, in a cumulative intrusion volume curve of particles, a first inflection point diameter is 0.30xc3x9710xe2x88x926 m or less, an interparticle void volume is 1.40xc3x9710xe2x88x923 to 2.20xc3x9710xe2x88x923 m3/kg and a particle void volume is 0.55xc3x9710xe2x88x923 to 0.80xc3x9710xe2x88x923 m3/kg.
In addition, the present invention is an annealing separator for a grain-oriented magnetic steel sheet, using the magnesium oxide particle aggregate having the above particle aggregation structure.
Further, the present invention is a grain-oriented magnetic steel sheet obtainable by a treatment using the above annealing separator.
In the present invention, a cumulative intrusion volume curve of particles means a curve which shows a relationship between a pore diameter and a cumulative pore volume determined from a pore distribution measurement by mercury porosimetry. FIG. 1 shows two cumulative intrusion volume curves of different types of magnesium oxide particle aggregates having different particle aggregation structures. A first inflection point is an inflection point at the largest pore diameter among inflection points at which the cumulative intrusion volume curve suddenly rises. It is indicated by a solid circle in the figure. A first inflection point diameter means a pore diameter at the first inflection point. An interparticle void volume means a cumulative pore volume at the first inflection point. A particle void volume means a volume obtained by subtracting the cumulative pore volume at the first inflection point from the total pore volume.